When transmitting a radio-frequency signal from a transmitter to a receiver in a radio channel, the signal propagates along one or more paths, in each of which the signal phase and amplitude vary, thus causing fades of different sizes in length and strength to the signal. In addition, the radio connection is also disturbed by noise and interference from other transmitters.
A radio channel can be tested either in actual conditions or in a simulator simulating actual conditions. Tests conducted in actual conditions are difficult, because tests taking place outdoors, for instance, are affected for example by the weather and season that change all the time. In addition, a test conducted in one environment (city A) does not fully apply to a second corresponding environment (city B). It is also usually not possible to test the worst possible situation in actual conditions.
However, with a device simulating a radio channel, it is possible to very freely simulate a desired type of a radio channel. In a digital radio channel simulator, the channel is modelled by a FIR (Finite Impulse Response) filter that forms a convolution between a channel model and a fed signal in such a manner that the signal delayed by different delays is weighted by channel coefficients, i.e. tap coefficients, and the weighted signal components are summed up. The channel coefficients are altered to reflect the behaviour of an actual channel.
Problems are, however, associated with using a FIR filter in a channel simulator. Calculation operations caused by multiplying by the channel coefficients and summing the delayed signals increase quadratically as a function of the number of FIR filter delay elements. Covering a long delay period by a large number of delay elements is thus not possible, because in the end it becomes impossible to perform the calculation sufficiently quickly. With a FIR filter, it is difficult to realize a delay that is not a multiple of the used delay unit. Such a delay can be achieved with an interpolation method, but forming the delay requires a great deal of calculation capacity. The delays of channel coefficients of signal components that propagated along different paths are not easy to float independently in a FIR filter employing direct convolution. A channel simulator is often also required to simulate a Doppler-shift. The extensive frequency-shifts of Doppler shifts are especially difficult to do in a FIR filter.